1. Field of the Invention
This invention relates to pressure sensors, and in particular, to pressure sensors suitable for use in high temperature environments, for example, on the order of 250.degree. C. to 300.degree. C.
2. Description of the Prior Art
Many micromechanical devices are now well known. Such devices include sensors of all types, for example, for sensing force, pressure, acceleration, chemical concentration, etc. These devices are termed "micromechanical" because of their small dimensions--on the order of a few millimeters square. The small size is achieved by employing photolithographic technology similar to that employed in the fabrications of integrated circuits. With this technology, the devices are almost as small as microelectronic circuits, and many such devices are fabricated in a batch on a single wafer or other substrate, thereby spreading the cost of processing that wafer among many individual devices. The resulting low cost and excellent performance enormously increases the applications for such devices. In addition, by forming such devices on a semiconductor substrate such as a silicon wafer, associated control and/or sensing circuitry may be formed on the same substrate during the same processes, thereby further increasing density and reducing cost.
At lease two types of silicon micromechanical pressure sensors are well known. For example, silicon capacitive and piezoresistive pressure sensors are described in "Silicon Micromechanical Devices," Scientific American (April 1983) 248(4):44-55, by Angel, Terry and Barth, one of the inventors herein. In capacitive silicon pressure sensors, a thin flexible diaphragm acts as one plate of a variable air gap capacitor. In piezoresistive sensors, electrical resistors are formed on or near the flexible diaphragm of a sensor, and change resistance when the diaphragm flexes. In the prior art, resistors in piezoresistive pressure sensors are formed by doping areas of the diaphragm and then providing electrical connections to the doped areas. When the diaphragm is flexed, mechanical stress in the resistors changes their electrical resistance. By placing four resistors in a Wheatstone bridge configuration, flexing of the diaphragm increases the resistance of two resistors and decreases the resistance of two resistors, thereby making the bridge more sensitive to pressure changes than if it were fabricated with a single resistor.
The resistors in such conventional silicon-based pressure sensors typically are formed by diffusing or ion-implanting a suitable impurity into the surface of the diaphragm region. For example, by implanting P-type impurity into an N-type diaphragm, the resistors are electrically isolated from each other by the resulting PN junctions. Unfortunately, the effectiveness of the PN junctions decreases as the temperature of the sensor increases. Above about 125.degree. C.-175.degree. C. the junctions are isolated so ineffectively because of junction leakage effects that it is difficult to obtain a reliable measurement of the resistances, thereby destroying the reliability of pressure measurements relying upon those resistance changes. The diffused resistors also suffer from the disadvantage that changes in PN junction depletion region width can change their resistance.
In an effort to increase the temperature capability of silicon pressure sensors, various resistor isolation techniques have been used. In one approach, rather than employing diffused resistors, deposited polycrystalline silicon resistors are employed. Unfortunately, the polycrystalline silicon does not have the same high value of piezoresistance coefficient as the single crystal silicon, thereby degrading the accuracy of pressure measurements. Additionally, monocrystalline silicon resistors are desirable because the polycrystalline resistors are not equally sensitive in all directions, and the grain boundaries are susceptible to stress problems at high temperatures.
Another approach has been to employ single crystal silicon resistors deposited on a glass layer or which are chemically affixed to the substrate, for example, using an organic bonding agent. Unfortunately, the glass and most organic agents soften at relatively low temperatures, and processes employing organic bonding are time consuming and expensive, resulting in resistors which protrude high above the surface of the diaphragm. The high resistors enhance the difficulty of effectively coupling the stresses from the diaphragm into the resistors. Furthermore, the glass or organic adhesives contain contaminants which can ruin other circuitry formed on the same die.
Another prior art technique has been to embed single crystal silicon resistors in a polycrystalline silicon substrate using a dielectric isolation process. Unfortunately, this technique means that the stresstransmitting membrane is not single crystal silicon, and is therefore subject to the undesirable mechanical properties of polycrystalline silicon.
Still another approach has been to form epitaxial silicon resistors on sapphire substrates. Unfortunately, sapphire is an expensive material which is difficult to machine into the complex geometries preferred for solid state pressure sensors.
References typifying the above techniques, as well as other approaches, are included in an accompanying disclosure statement.